The present invention relates to a MEMS or NEMS sensor with reduced noise.
The field of the invention is in particular that of body or surface force micro/nano-sensors working remotely, in particular inertial micro/nano-sensors, and more specifically accelerometers, or magnetic or electrostatic force sensors.
MEMS (microelectromechanical systems) or NEMS (nanoelectromechanical systems) accelerometers comprise a suspended seismic mass set in motion under the effect of an acceleration. “Piezoresistive” accelerometers also comprise a piezoresistive gauge sensitive to the movements of the mass and making it possible, by varying the resistivity, to determine the acceleration.
Document US 2007/0084041 describes an accelerometer implementing piezoresistive gauges, in which the mobile mass is suspended at the end of a clamped beam constituting a pivot link and the gauges extend parallel to the beam and are deformed during movement of the mass. This structure makes it possible to benefit from a lever arm effect, the force exerted on the piezoelectric gauges is then amplified relative to the force undergone by the suspended mass due to the acceleration by a factor dependent on the geometry of the system.
Accelerometers with piezoresistive gauges have the advantages of being inexpensive and easy to implement, and of requiring simple processing electronics. Nevertheless, they are difficult to use at low frequencies. Indeed, one type of noise, called 1/f, is predominant at low frequencies in the piezoresistive gauges and causes a very noisy response at low frequencies and bias drifts that are incompatible with certain applications. This is why, in many cases, capacitive accelerometers are preferred.
The noise mentioned above is called “1/f” because its spectral density is inversely proportional to the frequency.
The sources of noise in an accelerometer with detection by piezoresistive gauge are:                Brownian noise,        Johnson noise,        1/f noise, and        noise due to the measuring electronics.        
The predominant noise in piezoresistive accelerometers is 1/f noise, because it is integrated on a bandwidth close to the zero frequency.
1/f noise can limit the resolution. It can also create a bias drift.
In the document A. Barlian, “Review: Semiconductor Piezoresistance for Microsystems”, Proceedings of the IEEE, vol 97 (3), p 513-552, 2009, it is mentioned that the 1/f noise of the accelerometers can be reduced by acting on the parameters of the production method, such as the doping or the annealing temperature.